Bono says the recipe for a happy marriage is a little bit of mystery.

"After all these years she's still a mystery to me," he says about his wife Ali.

The pair, who celebrated Gavin Friday's 50th birthday at Carnegie Hall, have been together for over 30 years.

They began dating in 1975 and got married in 1982.

The couple appeared inseparable at the gig in Carnegie Hall and their marriage is often cited as a too-rare showbiz success.

Ali has often said that their marriage works because they work hard to keep it that way.

"Our marriage has worked because we like each other, because we talk to each other, and we are passionate about what we do. We allow each other to pursue our goals," she says.

"I wouldn't want to be married to someone who wasn't happy with what they were doing in life, and B (Bono) wouldn't either. I have learned a lot about what it means to be married, how great it can be if you persevere. We're very close. He says I'm very good with the dog whistle."